Developmental expression of the αIELβ7 integrin on T cell receptor γδ and T cell receptor αβ T cells

A novel monoclonal antibody, 2E7, was shown by immunoprecipitation to be reactive with the α_IELβ7 integrin and was employed to analyze the expression of this integrin in lymphocyte subsets and during T cell ontogeny. In adult lymph nodes, α_IEL was expressed at low levels by 40–70% of CD8⁺ T cells and < 5% of CD4⁺ T cells. However, virtually all intestinal intraepithelial lymphocytes and ?20% of lamina propria CD4⁺ T cells were 2E7⁺, indicating a preferential expression of this integrin on mucosal T cells. Examination of α_IEL integrin expression during thymus ontogeny revealed that ?3–5% of fetal or adult thymocytes were 2E7⁺. Interestingly, early in fetal thymus ontogeny, ?40% of 2E7⁺ cells expressed T cell receptor (TcR)-γδ and this subset persisted through birth. A developmental switch occurred such that 2E7⁺ TcR^? CD4^?8⁺ cells detected on fetal day 19 were followed by 2E7⁺ TcR-αβ CD4^?8⁺ cells in the neonatal thymus. The latter population persisted throughout thymus ontogeny into adulthood. Interestingly, a subset of TcR-γδ Vγ3⁺ day 16 fetal thymocyte dendritic epidermal cell (DEC) precursors were 2E7⁺, but all mature DEC expressed high levels of α_IEL integrin, suggesting that the α_IEL integrin was acquired late in DEC maturation. This possibility was strenghthened by immunohistochemical localization of the majority of 2E7⁺ γδ and αβ T cells to the medullary regions of the thymus. Overall, the results demonstrate a developmentally ordered expression pattern of the α_IELβ₇ integrin that suggests a common function for this integrin during TcR-γδ and -αβ CD4^?8⁺ T cell thymocyte development or perhaps in effector functions for these subsets.     

α4β1 integrin promotes accumulation of tissue‐resident memory CD8+ T cells in salivary glands

The salivary glands (SGs) of virus‐immune mice contain substantial numbers of tissue‐resident memory CD8⁺ T cells (T_RM cells) that can provide immunity to local infections. Integrins regulate entry of activated T cells into nonlymphoid tissues but the molecules that mediate migration of virus‐specific CD8⁺ T cells to the SGs have not yet been defined. Here, we found that polyinosinic‐polycytidylic acid (poly(I:C)) strongly promoted the accumulation of P14 TCR‐transgenic CD8⁺ T_RM cells in SGs in an α₄β₁ integrin‐dependent manner. After infection with lymphocytic choriomeningitis virus, accumulation of P14 T_RM cells in SGs and intestine but not in kidney was also α₄ integrin dependent. Blockade of α₄β₇ by monoclonal antibodies (mAbs) inhibited lymphocytic choriomeningitis virus‐induced accumulation of P14 T_RM cells in the intestine but not in SGs. In conclusion, our data reveal that α₄β₁ integrin mediates CD8⁺ T_RM accumulation in SGs and that poly(I:C) can be used to direct activated CD8⁺ T cells to this organ.     

α6 integrins participate in pro-T cell homing to the thymus

During embryogenesis, colonization of the thymic rudiment by hemopoietic progenitor cells depends on the adhesion of these cells to the jugular endothelium. Previously, we showed that progenitor T cells (pro-T cells) interact with α₆ integrins present on vascular endothelium. Here, we demonstrate that anti-α₆ integrin antibodies reduced the number of thymocytes up to 80 % in a congenic mouse model for thymus colonization by pro-T cells. In organotypic thymus cultures, the anti-α₆ integrin antibodies did not influence T cell development and proliferation. From this, we conclude that α₆ integrin participates in thymus homing. During mouse thymus ontogeny, α₆ integrin mRNA and protein expression was found as early as day 10 of development; at day 11, perithymic endothelial cells were α₆ integrin positive. Two α₆ integrin mRNA exist which are produced by alternative exon usage. The longer form, α₆, integrin, predominates during early embryonic stages, while the shorter α_6A form was present later during development. Although α₆, integrins can be displayed by immature thymocytes, strongest expression was found on intra- and perithymic vascular endothelium. These data suggest that α₆ integrins are involved in the homing of pro-T cells to the developing thymus by mediating adhesion of pro-T cells to the vascular endothelium.     

CCRL1/ACKR4 is expressed in key thymic microenvironments but is dispensable for T lymphopoiesis at steady state in adult mice

Thymus colonisation and thymocyte positioning are regulated by interactions between CCR7 and CCR9, and their respective ligands, CCL19/CCL21 and CCL25. The ligands of CCR7 and CCR9 also interact with the atypical receptor CCRL1 (also known as ACKR4), which is expressed in the thymus and has recently been reported to play an important role in normal αβT‐cell development. Here, we show that CCRL1 is expressed within the thymic cortex, predominantly by MHC‐II^lowCD40^? cortical thymic epithelial cells and at the subcapsular zone by a population of podoplanin⁺ thymic epithelial cells in mice. Interestingly, CCRL1 is also expressed by stromal cells which surround the pericytes of vessels at the corticomedullary junction, the site for progenitor cell entry and mature thymocyte egress from the thymus. We show that CCRL1 suppresses thymocyte progenitor entry into the thymus, however, the thymus size and cellularity are the same in adult WT and CCRL1^?/? mice. Moreover, CCRL1^?/? mice have no major perturbations in T‐cell populations at different stages of thymic differentiation and development, and have a similar rate of thymocyte migration into the blood. Collectively, our findings argue against a major role for CCRL1 in normal thymus development and function.     

Thymus‐resident memory CD8+ T cells mediate local immunity

The thymus is a primary lymphoid organ responsible for production and selection of T cells. Nonetheless, mature T cells and in particular activated T cells can reenter the thymus. Here, we identified memory CD8⁺ T cells specific for lymphocytic choriomeningitis virus or vaccinia virus in the thymus of mice long‐time after the infection. CD8⁺ T cells were mainly located in the thymic medulla, but also in the cortical areas. Interestingly, virus‐specific memory CD8⁺ T cells in the thymus expressed the cell surface markers CD69 and CD103 that are characteristic of tissue‐resident memory T cells in a time‐dependent manner. Kinetic analyses and selective depletion of peripheral CD8⁺ T cells by antibodies further revealed that thymic virus‐specific memory CD8⁺ T cells did not belong to the circulating pool of lymphocytes. Finally, we demonstrate that these thymus‐resident virus‐specific memory CD8⁺ T cells efficiently mounted a secondary proliferative response, exhibited immediate effector functions and were able to protect the thymus from lymphocytic choriomeningitis virus reinfection. In conclusion, the present study not only describes for the first time virus‐specific memory CD8⁺ T cells with characteristics of tissue‐resident memory T (T_RM) cells in a primary lymphoid organ but also extends our knowledge about local T‐cell immunity in the thymus.     

T-Cell Development in Early Partially Decapitated Chicken Embryos

We have evaluated the immunohistological and cytofluorometric changes that occur in the thymus of chicken embryos partially decapitated at 33-38 hr of incubation (DCx embryos) in an attempt to analyze possible neuroendocrinological influences on T-cell differentiation and, indirectly, the ontogeny of the so-called neuroendocrine-immune network. The thymus of DCx embryos shows important variations that profoundly and selectively affect different T-cell subsets, but not the nonlymphoid cell components of thymic stroma. These modifications include the accumulation of cell precursors, mainly DN (CD4^- CD8^-) cells and immature CD8^high CD4^- cells, which expand but do not differentiate, resulting in an extreme decline of both DP (CD4⁺ CD8⁺) cells and TcR c-expressing cells. Accordingly, both subcapsulary and outer cortex increase in size, whereas the deep cortex and principally the thymic medulla almost disappear in DCx embryos. In contrast, other T-cell subsets of DCx embryos, largely CDg^glowCD4^- cells and TcR γδ-expressing cells do not undergo significant variations throughout thymic ontogeny.     

Identification of preferential CD4+ T-cell targets for HIV infection in the cervix

A better understanding of the cellular targets of HIV infection in the female genital tract may inform HIV prevention efforts. Proposed correlates of cellular susceptibility include the HIV co-receptor CCR5, peripheral homing integrins, and immune activation. We used a CCR5-tropic pseudovirus to quantify HIV entry into unstimulated endocervical CD4⁺ T cells collected by cytobrush. Virus entry was threefold higher into cervix-derived CD4⁺ T cells than blood, but was strongly correlated between these two compartments. Cervix-derived CD4⁺ T cells expressing CD69, α₄β₇, or α₄β₁ were preferential HIV targets; this enhanced susceptibility was strongly correlated with increased CCR5 expression in α₄β₇⁺ and CD69⁺ CD4⁺ T cells, and to a lesser extent in α₄β₁⁺ CD4⁺ T cells. Direct binding of gp140 to integrins was not observed, integrin inhibitors had no effect on virus entry, and pseudotypes with an env that preferentially binds α₄β₇ still demonstrated enhanced entry into α₄β₁⁺ cells. In summary, a rapid and sensitive HIV entry assay demonstrated enhanced susceptibility of activated endocervical CD4⁺ T cells, and those expressing α₄β₇ or α₄β₁. This may relate to increased CCR5 expression by these cell subsets, but did not appear to be due to direct interaction of α₄β₇ or α₄β₁ with HIV envelope.     

FBXL12 regulates T‐cell differentiation in a cell‐autonomous manner

Aldehyde dehydrogenase (ALDH) activity is a hallmark of stem cells including embryonic, adult tissue and cancer stem cells. The SCF^FBXL12 complex is an authentic ubiquitin ligase that targets ALDH3 for degradation. FBXL12 is essential for the differentiation of trophoblast stem cells into specific cell types in the placenta during mouse embryogenesis, but its physiological functions in adult tissues have remained unknown. We have now investigated the role of the FBXL12‐ALDH3 axis in the thymus, in which FBXL12 was most abundant among adult mouse tissues examined. During T‐cell differentiation, FBXL12 is most abundant in CD4⁺CD8⁺ (DP) cells, with its expression declining as these cells differentiate into CD4⁺CD8^? or CD4^?CD8⁺ (SP) cells. T cells of FBXL12‐null mice manifested a differentiation block at the DP–SP transition that was associated with ALDH3 accumulation in DP cells. This differentiation block was also apparent in wild‐type mouse recipients of FBXL12‐null bone marrow transplants as well as in FBXL12‐null fetal thymic organ culture, suggesting that it is a cell‐autonomous phenomenon in the thymus rather than an indirect effect of altered systemic conditions. Our results thus indicate that, in addition to its role in placental development, the FBXL12‐ALDH3 axis is required for maturation of undifferentiated thymocytes.     

Neoplastic thymic epithelial cells of human thymoma support T cell development from CD4−CD8− cells to CD4+CD8+ cells in vitro

Human thymoma is a thymic epithelial cell tumour which often contains a large number of immature T cells and is frequently associated with autoimmune diseases. Since thymic epithelial cells play key roles in the development and selection of T cells in the normal thymus, we hypothesized that the neoplastic thymic epithelial cells of thymoma may support T cell differentiation in the tumour. We characterized CD4^?CD8^? cells in thymoma and applied an in vitro reconstitution culture system using the CD4^?CD8^? cells and the neoplastic epithelial cells isolated from thymoma. CD34, a stem cell marker, was expressed on 29.9 ± 12.2% of CD4^?CD8^? cells in thymoma. TCRγδ was expressed on 27.4 ± 15.1% of CD4^?CD8^? cells and CD19, a B cell marker, was expressed on 14.1 ± 23.1% of CD4^?CD8^? cells. CD4^?CD8^? cells expressed both IL-7R α-chain and common γ-chain. Purified CD4^?CD8^? cells from thymomas were cultured with the neoplastic epithelial cells, and their differentiation into CD4⁺CD8⁺ cells via CD4 single-positive intermediates was observed within 9 days' co-culture in the presence of recombinant IL-7. Furthermore, we examined the reconstitution culture using CD34⁺CD4^?CD8^? cells purified from normal infant thymus. The CD34⁺CD4^?CD8^? cells in normal thymus also differentiated to CD4⁺CD8⁺ cells in the allogeneic co-culture with the neoplastic epithelial cells of thymoma. These results indicate that the tumour cells of thymoma retain the function of thymic epithelial cells and can induce differentiation of T cells in thymoma.     

Both activated and nonactivated leukocytes from the periphery continuously enter the thymic medulla of adult rats: phenotypes,sources and magnitude of traffic

Although the thymus is primarily noted for the export of T cells to the periphery, a small influx of cells has also been observed. It is still a matter of debate whether entry into the thymus depends on prior activation. The phenotypes, sources and degree of immigration are largely unknown. We monitored by quantitative immunohistochemistry the entry of cells from the periphery into the rat thymus in three experimental models. We injected i.v. recirculating, small, nonactivated CD4⁺ T cell subsets, often referred to as naive (CD45RC⁺) and memory or antigen-experienced (CD45RC⁻) cells, purified from thoracic duct lymph of allotype-marked donors, allotype-marked leukocytes released from spleen or lung transplants, or leukocytes labeled in the periphery for 12 weeks during the S-phase of the cell cycle by oral application of 5-bromo-2-deoxyuridine (BrdUrd). Early after i.v. injection (0.5 h), significantly more antigen-experienced (CD45RC⁻) CD4⁺ T cells entered the thymus, and by 24 h four times as many cells from the CD45RC⁻ subset as from the CD45RC⁺ subset had entered the thymus and localized to the medulla. None of the thymic entrants expressed the interleukin (IL)-2 receptor. Following spleen transplantation ∼ 40% of donor cells entering the thymic medulla were T cells and ∼ 55% were B cells. In contrast, from a lung transplant, ∼ 85% of peripheral immigrants were T cells and ∼ 10% were B cells. After both procedures, a small number of NK cells and monocytes/macrophages were found among the immigrants (< 5%). Rats were fed BrdUrd continuously for 12 weeks, a procedure which labeled ∼ 30% of peripheral lymphocytes but not cortical thymocytes. BrdUrd-labeled cells were localized almost exclusively to the thymic medulla and represented ∼ 10% of medullary cells. Of the thymic immigrants ∼ 50% were T cells, ∼ 30% were B cells (including ∼ 15% IgD⁺ cells), ∼ 15% were NK cells and the remainder (∼ 5%) were monocytes/macrophages. Only a quarter of BrdUrd-labeled cells expressed the IL-2 receptor. The thymus is continuously infiltrated by both activated and nonactivated leukocytes from the periphery, including T cells, B cells, NK cells and monocytes. These immigrants are supplied by lymphoid and nonlymphoid organs in a characteristic subset composition. Their entry is facilitated by prior antigen experience or activation. Thus, the participation of the thymic medulla in general leukocyte traffic suggests a mechanism by which the T cell repertoire could potentially be modulated by the peripheral tissues.     

Expression of functionally active α4β1 integrin by thymic epithelial cells

We have investigated the expression and function of the VLA-4 heterodimer α₄β₁, a member of the β₁ integrin subfamily, on human thymic epithelial cells (TEC) derived from cortical epithelium. The expression of the α₄ integrin chain was studied in four different cloned TEC lines derived from either fetal or post-natal human thymus by both flow cytometry and immunoprecipitation techniques with anti-α₄ MoAbs. All different cell lines assayed expressed significant levels of α₄, as revealed by their reactivity with MoAbs specific for distinct α₄ epitopes. The α₄ subunit expressed by TEC was associated to β₁ but not to β₇ chain, and displayed the characteristic 80/70 kD pattern of proteolytic cleavage. The VLA-4 integrin in these cells was constitutively active in terms of adhesiveness to both fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In addition, this heterodimer localized to punctate regions of the cell in the area of contact with the substratum, named point contacts assessed by staining with the anti-β₁ activation epitope 15/7 MoAb. According to the cortical origin of the TEC lines expressing VLA-4, human thymus sections stained with different anti-α₄ antibodies revealed the presence of cortical, and in smaller numbers medullary epithelial cells bearing α₄ integrin. The expression of α₄ in the thymus was also found in both adult and fetal rats, in which epithelial cells were also specifically stained. Altogether, our data show that VLA-4 is an additional component of the integrin repertoire of TEC, and suggest that it could have an important role in thymus epithelial cell–thymocyte interactions.     

On the prenatal initiation of T cell development in the opossum Monodelphis domestica

Thymus‐dependent lymphocytes (T cells) are a critical cell lineage in the adaptive immune system of all jawed vertebrates. In eutherian mammals the initiation of T cell development takes place prenatally and the offspring of many species are born relatively immuno‐competent. Marsupials, in contrast, are born in a comparatively altricial state and with a less well developed immune system. As such, marsupials are valuable models for studying the peri‐ and postnatal initiation of immune system development in mammals. Previous results supported a lack of prenatal T cell development in a variety of marsupial species. In the gray short‐tailed opossum, Monodelphis domestica, however, there was evidence that αβT cells were present on postnatal day 1 and likely initiated development prenatally. Demonstrated here is the presence of CD3ε⁺ lymphocytes in late‐stage embryos at a site in the upper thoracic cavity, the site of an early developing thymus. CD3ε⁺ cells were evident as early as 48 h prior to parturition. In day 14 embryos, where there is clear organogenesis, CD3ε⁺ cells were only found at the site of the early thymus, consistent with no extra‐thymic sites of T cell development in the opossum. These observations are the first evidence of prenatal T cell lineage commitment in any marsupial.     

Preferential production of interferon-γ by CD4+ T cells expressing the homing receptor integrin α4/β7

Recent studies indicate that T helper type 1 (Th1) and 2 (Th2) lymphocytes differ in their expression of molecules that control T‐cell migration, including adhesion molecules and chemokine receptors. We investigated the relationship between cytokine production and expression of the homing receptor integrin α₄/β₇ on T cells. We began by analysing cytokine production by human CD4⁺ CD45RA^– memory/effector T cells following brief (4 hr) stimulation with phorbol 12‐myristate 13‐acetate (PMA) and ionomycin. α₄/ CD4⁺ T cells were more likely to produce the Th1 cytokine interferon‐γ (IFN‐γ) than were α₄/β₇^? CD4⁺ T cells in all six subjects studied. In contrast, production of the Th2 cytokine interleukin‐4 (IL‐4) was similar on α₄/ and α₄/β₇^? CD4⁺ T cells. In addition, we found that human CD4⁺ CD45RA^– T cells that adhered to the α₄/β₇ ligand mucosal addressin cell adhesion molecule‐1 (MAdCAM‐1) had a greater capacity to produce IFN‐γ than did non‐adherent cells, suggesting that the association between α₄/β₇ expression and IFN‐γ production has functional significance. These results suggested that primary activation under Th1‐promoting conditions might favour expression of α₄/β₇. We directly examined this possibility, and found that naïve murine CD4⁺ T cells activated under Th1‐promoting conditions expressed higher levels of α₄/β₇ compared to cells activated under Th2‐promoting conditions. The association between α₄/β₇ expression and IFN‐γ production by CD4⁺ T cells may help to determine the cytokine balance when MAdCAM‐1 is expressed at sites of inflammation in the intestine or elsewhere.     

Thymic emigration patterns in patients with type 2 diabetes treated with metformin

Recent data suggest that thymic output, which provides the naive T cells necessary for the normal functioning of T‐cell‐dependent immunosurveillance cellular immunity including anti‐cancer protection, can be disturbed in the course of type 2 diabetes. Metformin, an anti‐diabetic drug commonly confirmed as an agent with many potential anti‐cancer activities, might be helpful in this immune correction. The profile of thymic output was evaluated in the current study on the basis of the signal‐joint T‐cell receptor excision circle (sjTREC) concentration in peripheral blood polymorphonuclear cells and thymic emigrant content in peripheral blood evaluated from CD127 and/or CD132 antigen expression. It was revealed that recent thymic emigrants and more differentiated CD127⁺ CD132⁺ cell populations were decreased among naive T cells and CD8⁺ T cells, whereas RTE count was increased in CD4⁺ T cells, and the CD127⁺ CD132⁺ cell population was less numerous than in non‐diabetic participants. Terminally differentiated thymic emigrants, i.e. CD127^? CD132⁺ cells, were increased in naive T cells and in CD8⁺ T cells. Metformin affects mainly the early phases of thymic export, increasing CD127⁺ CD132^? and CD127⁺ CD132⁺ cell populations in naive T cells and the CD127⁺ CD132^? population in CD4⁺ T lymphocytes. It could be concluded that type 2 diabetes deteriorates thymic immunostasis. The decreased thymic output could be compensated by metformin, especially with regard to CD4⁺ naive T cells. It is the first time that therapy with metformin has been documented by us as particularly useful in the control and normalization of thymus function, regarding correction of early populations of thymic emigrants.     

Long-term thymic reconstitution by peripheral CD4 and CD8 single-positive lymphocytes

Significant immigration of peripheral T cells into SCID thymus was observed following reconstitution with normal Peyer's patch, mesenteric lymph node or peripheral lymph node cells. Immunohistologic and flow cytometric analyses reveal that T cells from these tissues are found in the thymus for as long as 177 days and can account for up to 67% of intrathymic cells. The returning cells express the CD3/Tcell receptor α/β complex, indicative of mature cells, and are equally divided among helper (CD4⁺ CD8 ) and cytotoxic (CD4^?/CD8⁺) phenotypes. The immigration of peripheral T cells is not accompanied by the appearance of immature, double-positive (CD4⁺CD8⁺) thymocytes as seen in similar reconstitutions using bone marrow. Taken together, these results suggest that peripheral T cells from a variety of lymphoid organs may regularly re-enter the thymus and, thus, possibly play a role in normal thymic development.     

Fibroblast dependency during early thymocyte development maps to the CD25+ CD44+ stage and involves interactions with fibroblast matrix molecules

We have investigated the role of specific components of the thymic stroma during development of CD4^? 8^? T cell precursors by separating and reaggregating precursor subsets with individual or combinations of stromal cells. We show that while the development of CD25⁺44⁺ precursors is dependent upon a combination of major histocompatibility complex (MHC) class II⁺ thymic epithelial cells and fibroblasts, their direct descendants, CD25⁺ 44^? precursors, develop to the CD4⁺8⁺ stage in the presence of MHC class II⁺ thymic epithelial cells alone. Thus, CD25⁺ 44⁺ precursors are the last developmental stage to be dependent upon fibroblast support. In addition, while metabolically inactive, 1-ethyl-3-(3'-dimethylaminopropyl)carbodiimide (ECDI)-treated fibroblasts retain the ability to promote T cell development, prior treatment with hyaluronidase abrogates this effect, suggesting that fibroblast-associated extracellular matrix components are the key elements involved. In support of this, we show that fibroblasts are located in cortical regions of the thymus where T cell precursors are known to reside, and that these fibroblasts are associated with an extensive extracellular matrix not found on thymic epithelial cells. Finally, antibodies to α4 integrin and CD44 interfere with the efficiency with which CD4⁺ 8⁺ cells are generated from CD25⁺ 44⁺ precursors in reaggregate cultures and also reduce the binding of the latter to 3T3 fibroblasts, suggesting these molecules play a role in bringing T cell precursors into contact with fibroblast-associated extracellular matrix.     

Tissue‐resident memory T cells: Local guards of the thymus

T‐cell surveillance of nonlymphoid tissues has traditionally been ascribed to recirculating memory T cells that continuously patrol the body. Extending this concept, recent evidence suggests that T cells also exist as nonmigratory memory cells that provide local immune protection in a broad range of peripheral tissues, including barrier locations such as skin and mucosa. In this issue of the European Journal of Immunology, Pircher and colleagues [Eur. J. Immunol. 2013. 43: 2295–2304] demonstrate, for the first time, the existence of such permanently tissue‐resident CD8⁺ memory T (T_RM) cells in a primary lymphoid organ, the thymus. T_RM cells in this location provide potent local immunity, which may help to preserve thymic integrity and normal T‐cell development in the face of infection with thymus‐invading pathogens.     

Changes in thymic export of γδ and αβ T cells during fetal and postnatal development

The thymus plays an essential role in the generation and selection of T cells and exports approximately 0.5–1% of thymocytes per day in young animals and considerably fewer in older animals. To date there have been no studies directly examining fetal thymic export in any species. Using the technique of intrathymic injection of fluorescein isothiocyanate, followed by an assay for green fluorescent cells in the periphery and for the expression of cell surface antigens on these cells, we have compared directly the export of T cells from the fetal and postnatal ovine thymus. While the thymus exports both αβ and γδ T cells, our results demonstrate that the proportion of thymic γδ T cells that are exported per day is much higher than that of thymic αβ T cells. Moreover, the export rate of γδ T cells increased from approximately 1 in every 60 γδ thymocytes per day emigrating from the fetal thymus to 1 in every 20 from the postnatal thymus. In addition, we identify a population of CD5⁺CD4^?CD8^?γδ^? T cells emigrating from the fetal thymus but greatly reduced among thymic emigrants after birth. These findings have several implications regarding the mechanisms and control of selection of both γδ and αβ T cells.     

Generation of specific effector and memory T cells with gut- and secondary lymphoid tissue- homing potential by oral attenuated CVD 909 typhoid vaccine in humans

Induction of effective memory T cells is likely to be critical to the level and duration of protection elicited by novel live oral typhoid vaccines. Using cells from volunteers who ingested Salmonella Typhi vaccine strain CVD 909, we characterized the induction of interferon (IFN)-γ-secreting central (T_CM, CD45RO⁺CD62L⁺) and effector (T_EM, CD45RO⁺CD62L⁻) memory T populations, and their gut-homing potential based on integrin α4/β7 expression. Both CD4⁺ T_EM and T_CM populations secreted IFN-γ. However, although CD4⁺ T_EM expressed, or not, integrin α₄/β₇, CD4⁺ T_CM cells were predominantly integrin α₄/β₇⁺. In contrast, IFN-γ-secreting CD8⁺ cells were predominantly classical T_EM and CD45RA⁺ T_EM (T_EMRA, CD45RO⁻CD62L⁻) subsets. However, although CD8⁺ T_EM expressed, or not, integrin α₄/β₇, CD8⁺ T_EMRA were predominantly integrin α₄/β₇⁺. This is the first demonstration that oral immunization of humans with S. Typhi elicits diverse IFN-γ-secreting CD4⁺ and CD8⁺ T_CM and T_EM subsets able to migrate to the gut and other lymphoid tissues.     

The immunoproteasome subunit LMP7 is required in the murine thymus for filling up a hole in the T cell repertoire

Cells of hematopoietic origin express high levels of the immunoproteasome, a cytokine‐inducible variant of the proteasome which has been implicated in regulating inflammatory responses and antigen presentation. In the thymus, medullary thymic epithelial cells (mTECs) and cortical thymic epithelial cells (cTECs) do express different proteasome subunits exerting chymotrypsin‐like activities suggesting distinct functions in thymic T cell selection. Employing the lymphocytic choriomeningitis virus (LCMV) infection model, we could show that the immunoproteasome subunit LMP7 was absolutely required for the generation of LCMV GP_118‐125‐specific T cells although the class I mediated presentation of GP_118‐125 was not dependent on LMP7. Using bone marrow chimeras and adoptive transfer of LMP7‐deficient CD8⁺ T cells into RAG1‐deficient mice we show that LMP7‐deficient mice lacked GP_118‐125‐specific T cell precursors and that LMP7 was required in radioresistant cells – most likely thymic epithelial cells ‐ to enable their selection. Since LMP7 is strongly expressed in negatively selecting mTECs but barely in positively selecting cTECs our data suggest that LMP7 was required to avoid excessive negative selection of GP_118‐125‐specific T cell precursors. Taken together, this study demonstrates that the immunoproteasome is a crucial factor for filling up holes within the cytotoxic T cell repertoire.